Microbiology and Biotechnology Research: An Overview Vol. 11

Extreme heat is becoming a recurrent rather than exceptional constraint on livestock production, health and welfare. Its biological effects extend well beyond depressed feed intake: thermal load changes heat exchange, endocrine and energetic metabolism, immune regulation, intestinal barrier function, microbial ecology, reproduction and development, while susceptibility differs markedly among species, breeds, life stages and production levels. This critical narrative review integrates evidence on five domains that are often treated separately: whole-animal physiology, immunity, gut and rumen microbiota, genetic thermotolerance, and precision detection and cooling. Literature published principally from 2010 through 5 July 2026 was appraised, with seminal earlier work retained where it established concepts still used in heat-tolerance phenotyping. The strongest causal evidence supports direct metabolic and intestinal consequences of hyperthermia that are not explained by reduced nutrient intake alone, and direct cooling remains the most immediately effective adaptation for intensively managed cattle. Immune responses are better characterised as dysregulation than uniform suppression because glucocorticoid-associated impairment can coexist with inflammatory activation, epithelial leakage and altered pathogen defence. Heat-associated microbiome shifts are biologically plausible and repeatedly observed, especially in poultry and increasingly in ruminants, but taxon-level signatures are inconsistent and causal intervention evidence remains weaker than evidence for barrier disruption. Genetic studies demonstrate heritable variation in heat response, genotype-by-environment interaction and the practical value of genomic selection; the SLICK phenotype provides a major-gene proof of concept, but polygenic selection and functional resilience phenotypes remain necessary. Precision livestock technologies can move heat management from environment-only thresholds towards animal-centred, anticipatory control, yet external validation, sensor interoperability, water and energy efficiency, and welfare-centred endpoints remain limiting. The evidence supports a layered climate-adaptation strategy in which genetics lowers baseline susceptibility, monitoring detects individual thermal strain, targeted cooling prevents damaging hyperthermia, and nutritional or microbiome interventions are used as complementary rather than primary controls.